Heretofore compressed powder magnetic cores, which are formed in predetermined shapes by binding ferromagnetic powder such as iron, iron alloy, or ferrite with binder resin such as epoxy resin, etc., are well known (e.g. JP-B-47-22514, JP-B-50-14207, JP-A-49-4197, etc.).
Compressed powder magnetic cores of this kind are usually either formed by compressing molding filling a die, after having mixed ferromagnetic powder such as iron, iron alloy, ferrite, etc. with binder resin such as epoxy resin, or formed by compressing molding, filling a die, after having coated ferromagnetic powder with binder resin such as epoxy resin, etc. A fabrication method was used, by which this compact thus formed by compressing molding is taken out from the die and subjected to heating hardening in a thermostatic oven to fabricate a compressed power magnetic core.
In contrast to the prior art described above, a first problem, which this invention is going to solve, is as follows.
In this kind of compressed powder magnetic cores their magnetic characteristics depend on the density of the ferromagnetic powder in the compact and the mixing ratio of the ferromagnetic powder and the binder resin has important influences on the magnetic characteristics. For this reason, according to the prior art techniques, for a compressed powder magnetic core formed by mixing ferromagnetic powder and binder resin and by compressing molding, the content ratio (mixing ratio) of the composition was defined by the volume % or the weight % of the binder resin and the ferromagnetic powder, occupying the compact.
However, in the binder resin within the compressed powder magnetic core after the molding usually void is formed, after solvent, etc. contained in the binder resin have been vaporized. The density of the whole compact is varied, depending on the void ratio in the whole compact, which varies the density of the ferromagnetic powder in the compact. For this reason, magnetic characteristics of the compressed powder magnetic core differ from those originally expected which gives rise to a problem. However for the compressed powder magnetic core according to the prior art techniques, since no attention is paid to the void ratio and the mixing ratio is defined only by the content ratio in the compact made of the binder resin and the ferromagnetic powder, even if the quantity of the binder resin and that of the ferromagnetic powder are constant, the void ratio varies, depending on differences in the pressure, etc. at the molding, and it may have significant influences on the magnetic characteristics of the compressed powder magnetic core which gives rise to a problem.
For example, in the case where the weight % of the binder resin is 30 vol % (volume %) and the compressed powder magnetic core is molded, varying the compacting pressure, the void ratio varies, depending on the compacting pressure, as indicated in Table 1. As the result, the density of the ferromagnetic powder in the compact varies and therefore the magnetic characteristics thereof vary.
Consequently in compressed powder magnetic cores fabricated as by the prior art techniques, paying attention only to the mixing ratio of the binder resin and the ferromagnetic powder, fluctuations in the magnetic characteristics take place, depending on differences in the void ratio, even for compressed powder magnetic cores formed with a same mixing ratio which gives rise to a drawback that the reliability as a product is low.
TABLE 1 ______________________________________ SATURATION COMPACT- MAGNETIC SAM- ING VOID FLUX PLE BINDER PRESSURE RATIO DENSITY ______________________________________ A 30 (vol %) high low high B 30 (vol %) low high low ______________________________________
A second problem which this invention is going to solve, is as follows.
By the prior art method for fabricating the compressed powder magnetic core described previously, there were drawbacks that air bubbles, which have entered it at mixing and at filling a die, voids after solvent in the binder resin has been vaporized, etc. are apt to remain as they are and that told portions remain relatively abundantly within the fabricated compressed powder magnetic core.
For this reason compressed powder magnetic cores fabricated by the prior art method had a drawback that the robustness thereof is low because of the presence of void therein and therefore they are easily destroyed. Further they had a problem that the filling ratio in the compact with ferromagnetic powder (density of the ferromagnetic powder in the compact) and the saturation magnetic flux density are low because of the presence of the void and therefore the magnetic characteristics are apt to be unsatisfactory.
A third problem, which this invention is going to solve, is as follows.
In a prior art compressed powder magnetic core ferromagnetic powder having a relatively small grain size was used as ferromagnetic powder, which is in charge of the magnetic characteristics, in order to prevent eddy current loss and to try to improve the property of the compressed powder.
However there was a problem that, in the case where ferromagnetic powder having a small grain size is used, the fluidity of the compound of the ferromagnetic powder and the binder resin is reduced and difficulties such as bad pouring into a die, mixing of bubbles, etc. are apt to take place, which gives rise bad filling of the die.
For this reason, a compressed powder magnetic core according to the prior art techniques had a problem that in the case where it is formed by using ferromagnetic powder having a small grain size, a large void portion is apt to be formed in the compact after the compressing molding, which reduces the density of the compact and lowers the mechanical strength and the magnetic characteristics of the compressed powder magnetic core.
A fourth problem, which this invention is going to solve, is as follows.
The prior art compressed powder magnetic core had a drawback that since iron and iron alloys are used as the magnetic material, red rust is apt to be produced in an environment of high temperature and high humidity.
In particular, at the surface of the compressed powder magnetic core the resin coating layer is apt to peel due to wear at the compressing molding, scratch after the molding, etc. and the surface of the magnetic powder made of iron or iron alloy is often exposed, which accelerates the production of red rust.
Further, in a compressed powder magnetic core, void is apt to remain within the compact after the compressing molding. For this reason, there was a problem that water penetrates in the void, which may cause the production of red rust.